1. Field of the Invention
This invention relates generally to computed tomographic (CT) scanners and, more particularly, to CT scanners that use electron rastered x-ray generators or other means of projecting an electron beam to different positions on the anode.
2. Description of the Related Art
For many applications, CT is currently a preferred method for scanning objects using x-rays. CT allows a two- or three-dimensional image of the interior of an object to be constructed without physical intrusion into the object. For example, CT can be used to develop a three-dimensional image of the contents of baggage without physically opening the baggage, as would be useful in airport security screening. CT scanners are also used for medical applications (for example to scan different parts of the body), in industrial applications (for example in rapid prototyping and reverse engineering), and for non-destructive testing (for example to detect defects of turbine blades and ceramics).
The fundamental principles of CT are well known. In one approach, a fan of x-rays is projected from a source point through the object onto a detector array. Each beam within the fan propagates through the object along a different trajectory in a scan plane and the detector element that collects that beam therefore collects information about the object along the trajectory. The information collected from propagation of the x-ray fan shall be referred to as a view. This process is repeated for many different source points (the locus of source points shall be referred to as the source path), generating multiple views of the object. The views are then combined using various reconstruction techniques, such as filtered back projection, to produce a two-dimensional image of the object. Most reconstruction algorithms require that each point to be reconstructed be traversed by x-rays propagating along all different angles. Rays 180 degrees apart are redundant, so in practice a set of x-rays spanning 180 degrees (rather than a full 360 degrees) is sufficient. The set of views that together provide rays of 180 degrees for each point is referred to as a full set of views. The source path of the scanner preferably is sufficient to provide a full set of views (either directly or through interpolation or other techniques).
The basic process for producing two-dimensional images can be extended to produce a three-dimensional image of the object. In a common approach, a full set of views is acquired at different “depths” along the object, each full set of views is used to produce a two-dimensional slice of the object at the corresponding depth, and the slices are then pieced together to form a three-dimensional image. This slice-based approach is generally preferred for industrial CT systems. In another common approach, views are acquired as the object is continuously translated relative to the CT scanner. As a result, rather than acquiring a full set of views at a fixed depth coordinate (as is the case in the slice-based approach), each view is acquired at a slightly different depth coordinate due to the object translation and interpolation can be used to fill in gaps as necessary. This method is often referred to as spiral or helical scanning and is generally preferred for medical applications. Regardless of whether slice or spiral scanning is used, the resulting three-dimensional images can be displayed using a wide range of conventional graphics techniques.
One drawback to conventional CT systems is their slow scanning speed for some applications. The number of views required for a full set and the number of full sets required per object depend on the desired resolution. For even moderate applications, a large number of views may be required to generate satisfactory images of an object. As a result, if views cannot be generated quickly enough, the time required to scan an object will be unacceptably long.
In a common conventional approach, a single x-ray source is used to project an x-ray fan from a source point to a detector array. The x-ray source and the detector array are mounted on a gantry on opposite sides of the object. The entire gantry is mechanically rotated about the object to generate the required views. At a typical rotation speed of two revolutions per second, several seconds may be required to scan a single object. If the objects are luggage being screened, for example, this typically results in a throughput of less than 500 bags per hour.
The inspection time can be reduced in a number of ways. In one conventional approach, the gantry is rotated at a higher frequency. However, the gantry can be large and rotating it at high speeds can introduce its own problems, not the least of which is the increased centrifugal force exerted on the gantry. The rotational frequency can be reduced by acquiring several views simultaneously using multiple detector arrays (e.g., multi-slice detectors). However, this requires multiplying the number of detectors that are used. For example, acquiring two views simultaneously typically requires twice as many detectors. This adds cost and complexity to the system.
U.S. Pat. No. 4,352,021 (Boyd, et al.) suggests another approach for generating views. This approach uses a stationary, semi-circular x-ray target that encompasses the object. An electron beam is scanned across the entire x-ray target. One scan of the electron beam across the entire x-ray target traverses the entire source path and generates a full set of views. The electron beam scanning is faster than mechanical rotation of a gantry so higher throughput can be achieved. However, the semi-circular x-ray target may be several feet in diameter for many common applications. As a result, a large electron beam assembly, accurate devices for deflecting the electron beam, and long propagation paths for the electron beam are typically required. This assembly also makes it difficult to scan objects on a conveyor belt because the tube assembly occupies a large volume on one end of the scanner. The long propagation paths also result in a large overall size for the equipment and increased sensitivity to electromagnetic noise, making it inappropriate for use in industrial environments such as airports. The large assembly, which must be kept under vacuum, is also difficult to make leak proof. In order to maintain vacuum, a vacuum pump system is required. The high scanning speed requires a very high tube power in order to achieve a useful x-ray flux.
The Boyd design also has the drawback that the arc of the x-ray source overlaps the arc of the detectors (i.e., source arc+detector arc is greater than 360 degrees), which means that the detector arc and source arc must be displaced in the depth direction. The angle from source to detector with respect to the plane defined by the source arc is referred to as the cone angle. The cone angle, if uncorrected, leads to inaccurate image reconstruction. U.S. Pat. No. 6,735,271 (Rand, et al.) teaches improvements of the Boyd machine by using an x-ray target and detector that are segments of a helix instead of segments of circles. The detector array and target arc are arranged such that active portions of the source and detector are diametrically opposed to each other. This design also provides multislice scanning of an object that is in constant motion at a critical velocity without having to interpolate spiral data.
In another suggested approach, a number of conventional x-ray tubes are positioned around the object and are then turned on and off in sequence. Turning on one x-ray tube activates one source point, generating one x-ray fan and one view of the object. Turning on the next x-ray tube would generate the next view, and so on. However, for many applications, conventional x-ray tubes cannot be spaced close enough to one another to meet the application's resolution requirements.
Some researchers are conducting research into small x-ray generators (nano-tubes). If these devices can be made sufficiently small, then a large number of them can be positioned around the object with an adequate spacing to obtain a high spatial resolution. However, this technology does not appear to be advanced enough for commercial application at this time at voltages greater than 60 kV, which makes this approach inappropriate for medical, baggage scanning and many industrial applications that may require significantly greater x-ray energies. This approach also requires complex systems to distribute power and cooling to the large number of x-ray tubes.
Thus, there is a need for CT scanners that are fast, can provide good spatial resolution, have the capability to scan a large aperture, are cost effective, and/or are able to withstand industrial environments with a compact footprint.